The free viewportcenter.lsp has been updated with more functionalities.
This app shows the coordinates for the center of the current viewport on command line.
If command is run within a viewport in model space it can be run transparently for example like this using 'vpc
Command: LINE
Specify first point: 'vpc
If command is in paper space it will ask to select viewport and the point will be available as last used point and can be used with @ like this:
Command: LINE
Specify first point: @